This invention relates to earth soil character measurements and more particularly to a method for electronically determining the earth's crust character and its effect on transmissivily therethrough of seismic signals, using a passive seismic transducer as the sensing device.
The system operates on the principle that an earth's background noise signal is present at any given location. The background noise signal may be derived from a variety of random background sources, singly and/or collectively, such as rain, seashore wave activity, wind, miscellaneous far distant land or air traffic activity etc., the signal of which is measured as background noise resonance frequency. This noise frequency is known to vary for different geological locations. The small signals generated by natural and cultural background are passed by the cover of the earth's surface which acts as a sounding board for small signals. Field testing has shown that these small signals predominate when no other signal sources are within detection range of a Geophone. This system takes these low level signals, amplifies them, and determines what the noise frequency of the earth is at that geological location. The noise frequency of the earth at a given location is directly related to the geological constuction or formation of the earth at that location; i.e., the thickness and material of the first layer of soil.
In so far as applicants are aware, the technique or method of using seismic signatures to determine the approximate characteristics of the earth's crust is a new concept. Applicants are aware of seismic prospecting systems such as the system disclosed in U.S. Pat. No. 3,221,297 to Smith et al. In systems such as disclosed in the said Smith et al patent, a composite seismic signal is obtained by recording, individually, the reflections of a number of generated single frequency signals. Such systems do not use the seismic signature of the earth to determine the resonance or background noise frequency of the earth, and are not concerned with the thickness of the first layer of soil. Applicants are further aware of U.S. Pat. No. 4,158,832, to Barnes et al dated June 19, 1979, which discloses a specific seismic apparatus embodying electronic circuitry to purportedly identify the presence and classify the type of vehicle source generating a seismic signature. The present electronic method and novel applicable use is very helpful to enhance the system of said Barnes et al U.S. Pat. No. 4,158,832 by making it truly effective irrespective of in which geographical area it is attempted to be used.